This invention relates generally to magnetic tunnel junction (MTJ) heads. More particularly, it relates to MTJ heads with flux guides defining the track widths of MTJ heads.
Magnetic tunnel junction (MTJ) devices based on the phenomenon of spin-polarized electron tunneling. A typical MTJ device includes two ferromagnetic layers separated by a thin insulating tunnel barrier layer. One of the ferromagnetic layers has a higher saturation field in one direction of an applied magnetic field, typically due to its higher coercivity, than the other ferromagnetic layer. The insulating tunnel barrier layer is thin enough that quantum mechanical tunneling of electrons can occur between the ferromagnetic layers. The tunneling phenomenon is electron-spin dependent, making the magnetic response of the MTJ a function of the relative orientations and spin polarizations of the two ferromagnetic layers.
MTJ devices have been proposed primarily as memory cells for solid state memory devices. The state of the MTJ memory cell is determined by measuring the resistance of the MTJ when a sense current is passed perpendicularly through the MTJ from one ferromagnetic layer to the other. The probability of tunneling of charge carriers across the insulating tunnel barrier layer depends on the relative alignment of the magnetic moments (magnetization directions) of two ferromagnetic layers. The tunneling current is spin polarized, which means that the electrical current passing from one of the ferromagnetic layers, for example, a layer whose magnetic moment is fixed or prevented from rotation, is predominantly composed of electrons of one spin type (spin up or spin down, depending on the orientation of the magnetic moment of the ferromagnetic layer). The degree of spin polarization of the tunneling current is determined by the electronic band structure of the magnetic material comprising ferromagnetic layer at the interface of the ferromagnetic layer with the tunnel barrier layer. The first ferromagnetic layer thus acts as a spin filter. The probability of tunneling of the charge carriers depends on the availability of electronic states of the same spin polarization as the spin polarization of the electrical current in the second ferromagnetic layer. Usually, when the magnetic moments of the first and second ferromagnetic layers are parallel to each other, there are more available electronic states than when the magnetic moments of the two ferromagnetic layers are aligned antiparallel to each other. Thus, the tunneling probability of the charge carriers is highest when the magnetic moments of both layers are parallel, and is lowest when the magnetic moments are antiparallel. When the moments are arranged neither parallel nor antiparallel, the tunneling probability takes on an intermediate value. Thus, the electrical resistance of the MTJ memory cells depends on the spin polarization of the electrical current and the electronic states in both of the ferromagnetic layers.
MTJ heads have attracted more attention since a large tunneling magneto-resistance (TMR) was found at room temperature. These MTJ heads with TMR larger than 40% are potentially applicable as magnetoresistive read heads for high areal density recording. However, prior to head application, several design issues must be worked out. These issues include shield to shield distance, longitudinal biasing, and shorting across the insulating tunnel barrier during mechanical lapping processes. The shorting problem is particularly important since the head typically undergoes several lapping processes during the definition of the air bearing surface (ABS). More importantly, future higher areal density data storage requires that MTJ heads be able to operate at ever-decreasing track widths (TW). However, the cross-sectional area of an MTJ head decreases as the track width decreases. Generally, the resistance of an MTJ head depends inversely on the cross-sectional area of the MTJ valve. The resistance of an MTJ head can become very high for a sufficiently small cross-sectional area. A larger resistance generally means a larger noise level in the MTJ head, which leads to a poor signal to noise ratio (SNR).
U.S. Pat. No. 5,898,547 issued Apr. 27, 1999 and U.S. Pat No. 5,901,018 issued May 4, 1999 to Fontana, Jr. et al. disclose a magnetic tunnel junction (MTJ) magnetoresistive read head having an MTJ sensing or free ferromagnetic layer that functions as a rear flux guide to direct magnetic flux from a magnetic recording medium to the tunnel junction. The back edge of the free layer is located farther than the back edges of the tunnel barrier layer and the pinned layer from a sensing surface of the MTJ heads. Unfortunately, the track widths of the MTJ heads have not been reduced to optimize the efficiency of the MTJ heads. Furthermore, Fontana et al. does not teach the prevention of electrical shorts during the definition of air bearing surface (ABS).
U.S. Pat No. 5,930,087 issued Jul. 27, 1999 to Brug et al. discloses a robust recording head with a spin tunneling sensing element separated from an interface between the recording head and a recording media to prevent collisions and other ill effects at this interface. The spin tunneling sensing element includes a pair of magnetic elements, wherein one of the magnetic elements functions as a flux guide that conducts magnetic flux emanating from a recording medium away from the interface to an active area of the spin tunneling sensing element. However, Brug does not teach the use of a flux guide for reducing track width to optimize the efficiencies of the robust recording heads.
Japanese published patent application JP8-115511, published May, 7, 1996 discloses the use of a flux guide to improve the signal to noise ratio of a spin valve type GMR head by increasing a signal level from the spin valve as the track width is reduced. However, this publication does not address MTJ heads. Furthermore, the publication does not address reducing the noise level.
An article entitled xe2x80x9cEvaluation of Front Flux Guide Type Magnetic Tunnel Junction Headsxe2x80x9d published April 2000 in INTERMAG 2000 Conference Digest to Shimazawa et al. discloses magnetic tunnel junctions (MTJs) heads using the free layers as flux guide to prevent the electrical short during the definition of the air bearing surface (ABS). Unfortunately, Shimazawa et al. does not teach the use of a flux guide for reducing the track width so that the efficiencies of the MTJs heads are optimized.
Another article entitled xe2x80x9cThe Electrical and Magnetic Response of Yoke-Type Read Heads Based on A Magnetic Tunnel Junctionxe2x80x9d by Coehoorn et al. and published IEEE Transactions on Magnetics, vol. 35, No. 5, September 1999, discloses yoke-type read heads containing a tunnel junction magnetoresistive element (TMRE) that includes a front upper flux guide, a back upper flux guide, and a bottom flux guide. Coehoorn et al. does teach the use of the front flux guide with an anisotropic permeability to minimize side reading for a small track width, but does not teach the use of the flux guide for reducing the resistance to increase areal density of magnetic storage media.
There is a need, therefore, for an MTJ head including a flux guide that overcomes the above difficulties.
Accordingly, it is a primary object of the present invention to provide MTJ heads with flux guides that prevent electrical shorting of the MTJ layers during the definition of air bearing surface.
It is a further object of the invention to provide MTJ heads with flux guides that reduce the track widths while leaving the cross-sectional areas unchanged. Therefore, the resistance of the tunnel junction valve is reduced, which reduces noise and improves the signal to noise ratio.
It is an additional object of the invention to provide MTJ heads with flux guides to prevent the leakage of the magnetic flux into the shields of the MTJ sensors.
It is another object of the invention to provide MTJ heads with flux guide enhancing the flux coupling to the MTJ sensor.
It is another object of the present invention to provide a MTJ heads have high efficiencies.
It is a further object of the present invention to provide a method for producing MTJ heads having above properties.
These objects and advantages are attained by MTJ heads having flux guides proximate the edges of MTJ valves.
According to a first embodiment of the present invention, a MTJ head includes a MTJ valve and a first flux guide proximate the first edge of the MTJ valve. An insulating layer is located between the MTJ valve and the first flux guide for electrical insulation between these layers. The first flux guide has a first portion, which defines the track width of the MTJ head, proximate an air bearing surface of the MTJ head, and a second portion proximate the first edge. The width of the first portion, which defines the track width, is smaller than the width of the second portion, the sensor width. As a result, the cross-sectional area of the MTJ valve is not decreased when the track width is reduced. Therefore, the resistance of the MTJ head is lower, which optimizes the signal to noise ratio of the MTJ head by reducing a noise level of the MTJ valve.
The MTJ head further includes a second flux guide proximate a second edge of the MTJ valve, which is farther from the air bearing surface than the first edge. A second insulating layer is located between the second flux guide and the MTJ valve for electrical isolation. The MTJ head also includes two shields that act as the electrical leads. The first and the second flux guides are electrically insulated from these shields.
The MTJ head also includes two hard bias layers disposed on both sides of the MTJ valve and the first and second flux guides for biasing the MTJ valve and the flux guides. Two other insulating layers are disposed between the hard bias layers and the MTJ valve and the first and second flux guides for electrical isolation. Two hard bias layers are also insulated from the shields. The first and second flux guides are typically made of magnetically soft materials such as permalloys of Co, Ni, and Fe. The insulating layers are typically made of Al2O3, AlN, or MgO.
An MTJ valve typically includes a ferromagnetic free layer, a ferromagnetic pinned layer, and an insulating barrier layer disposed between the free layer and the pinned layer for permitting tunneling current in a direction generally perpendicular to the planar pinned and free layers. Since the free and pinned layers are very thin, there is a risk of an electrical short between these two layers due to smearing during the polishing the air bearing surface. By using the first flux guide the electrical short is prevented since the surface of the flux guide is polished as opposed to the edges of MTJ valve layers. Furthermore, a flux guide with an appropriate height may be used to prevent some of the magnetic flux from leaking into the shields.
MTJ heads of the type depicted in the first embodiment are made by a method according to a second embodiment of the present invention. A MTJ valve is made by sputtering or vacuum deposition on a first shield of the layers at a wafer level. Material is selectively removed from the wafer to define the MTJ valve. A first insulating layer and a second insulating layer are deposited proximate a first and a second edge of the MTJ valve such that the second edge is farther from an air bearing surface of the MTJ head than the first edge. A first flux guide and a second flux guide are deposited adjacent to the first and the second insulating layers respectively. The first flux guide has a defined track width, which is smaller than the width of the MTJ valve.
Two insulating layers are deposited on two other sides of the MTJ valve before two hard bias layers are deposited in proximity to these two insulating layers. The MTJ valve and the first and second flux guides are located between these two hard bias layers. A second shield is then deposited such that the first and second shields sandwich the MTJ valve. Third and fourth insulating layers are deposited to electrically insulate the first and second flux guides from the second shield. Furthermore, two other insulating layers are also deposited between the hard bias layers and the second shield to electrically insulate the hard bias layers from the second shield.
The MTJ heads of the type depicted in the first and second embodiments may be incorporated into a disk drive according to a third embodiment of the present invention. A disk drive system includes a magnetic recording disk connected to a motor, a MTJ head connected to an actuator. The actuator moves the MTJ head across different regions in the magnetic recording disk, and the motor spins the magnetic recording disk relative to the MTJ head.
MTJ heads and disk drives made according to the various embodiments of the present invention exhibit reduced track widths while leaving the cross-sectional area of the MTJ unchanged. Therefore, the resistance of the MTJ head is reduced, and the signal to noise ratio is improved.